The invention relates to making interconnections between electronic components, especially microelectronic components and, more particularly, to providing techniques for removably mounting (socketing) semiconductor dies and packages to circuit boards.
Electronic components, particularly microelectronic components such as semiconductor devices (chips), often have a plurality of terminals (also referred to as bond pads, electrodes, or conductive areas). In order to assemble such devices into a useful system (or subsystem), a number of individual devices must be electrically interconnected with one another, typically through the intermediary of a printed circuit (or wiring) board (PCB, PWB).
Semiconductor devices are typically disposed within a semiconductor package having a plurality of external connection points in the form of pins, pads, leads, solder balls, and the like. Many types of semiconductor packages are known, and techniques for connecting the semiconductor device within the package include bond wires, tape-automated bonding (TAB) and the like. In some cases, a semiconductor device is provided with raised bump contacts, and is connected by flip-chip techniques onto another electronic component.
Generally, interconnections between electronic components can be classified into the two broad categories of xe2x80x9crelatively permanentxe2x80x9d and xe2x80x9creadily demountablexe2x80x9d.
An example of a xe2x80x9crelatively permanentxe2x80x9d connection is a solder joint. Once two components are soldered to one another, a process of unsoldering must be used to separate the components. A wire bond is another example of a xe2x80x9crelatively permanentxe2x80x9d connection.
An example of a xe2x80x9creadily demountablexe2x80x9d connection is rigid pins of one electronic component being received by resilient socket elements of another electronic component. The socket elements exert a contact force (pressure) on the pins in an amount sufficient to ensure a reliable electrical connection therebetween. Interconnection elements intended to make pressure contact with an electronic component are referred to herein as xe2x80x9cspringsxe2x80x9d or xe2x80x9cspring elementsxe2x80x9d.
Spring elements are well known, and appear in a variety of shapes and sizes. In today""s microelectronic environment, there is a profound need for all interconnection elements, including springs, to become smaller and smaller, in order that a large plurality of such interconnection elements can be disposed in a small area, to effect a high density of interconnections to electronic components.
Prior art techniques for making spring elements generally involve stamping (punching) or etching a spring material, such as phosphor bronze or beryllium copper or steel or a nickel-iron-cobalt (e.g., kovar) alloy, to form individual spring elements, shaping the spring elements to have a spring shape (e.g., arcuate, etc.), plating the spring elements with a good contact material (e.g., a noble metal such as gold, which will exhibit low contact resistance when contacting a like material) and molding a plurality of such shaped, plated spring elements into a linear, a peripheral or an array pattern. When plating gold onto the aforementioned materials, sometimes a thin (for example, 30-50 microinches), barrier layer of nickel is appropriate.
Various problems and limitations are inherent with such techniques of making spring elements.
For example, these processes are limited when applications demand that a plurality of springs (interconnection elements) be arranged at a fine (e.g., 10 mil) pitch. Such a fine pitch inherently demands that each spring be sized (i.e., in cross-section) substantially smaller (e.g., 3 mil) than the pitch. A punch-out area must be accommodated, and will limit how much material is left over to form springs. At best, even through it may be relatively straightforward to punch out springs as small as 1 mil, such small sizes impose limitations on the contact force that can reliably be exerted by the springs. This is especially poignant in the context of fabricating area arrays of springs.
Generally, a certain minimum contact force is desired to effect reliable pressure contact to electronic components (e.g., to terminals on electronic components). For example, a contact (load) force of approximately 15 grams (including as little as 2 grams or less and as much as 150 grams or more, per contact) may be desired to ensure that a reliable electrical connection is made to a terminal of an electronic component which may be contaminated with films on its surface, or which has corrosion or oxidation products on its surface. The minimum contact force required of each spring demands either that the yield strength of the spring material or that the size of the spring element are increased. As a general proposition, the higher the yield strength of a material, the more difficult it will be to work with (e.g., punch, bend, etc.). And the desire to make springs smaller essentially rules out making them larger in cross-section.
Another limitation attendant prior art interconnection elements is that when hard materials (such as would be used for making springs) are employed, relatively xe2x80x9chostilexe2x80x9d (e.g., high temperature) processes such as brazing are required to mount the interconnection elements to terminals of an electronic component. For example, it is known to braze rigid pins to 30 relatively xe2x80x9cdurablexe2x80x9d semiconductor packages. Such xe2x80x9chostilexe2x80x9d processes are generally not desirable (and often not feasible) in the context of certain relatively xe2x80x9cfragilexe2x80x9d electronic components such as semiconductor devices. In contrast thereto, wire bonding is an example of a relatively xe2x80x9cfriendlyxe2x80x9d processes which is much less potentially damaging to fragile electronic components than brazing. Soldering is another example of a relatively xe2x80x9cfriendlyxe2x80x9d process.
Another problem associated with mounting springs on electronic components is largely mechanical in nature. In cases where a spring is mounted at one end to a substrate (which, for purposes of this proposition is considered to be an immovable object), and is required to react forces applied at its free end, the xe2x80x9cweak linkxe2x80x9d (weakest point, in service) will often be the point at which the spring is attached (e.g., the base of the spring is bonded) to the substrate (e.g., terminal of an electronic component). This accounts, at least in part, for the requirement to employ xe2x80x9chostilexe2x80x9d processes (e.g., brazing) to mount the springs to the substrate.
Another subtle problem associated with interconnection elements, including spring contacts, is that, often, the terminals of an electronic component are not perfectly coplanar. Interconnection elements lacking in some mechanism incorporated therewith for accommodating these xe2x80x9ctolerancesxe2x80x9d (gross non-planarities) will be hard pressed to make consistent contact pressure contact with the terminals of the electronic component.
In many modern electronic systems, one or more packaged semiconductor devices are mounted to circuit boards. Various packaging types are well known. Generally, all semiconductor packages have external connections which are either pins, pads, leads, ball bumps, or the like.
One type of semiconductor package is typified by U.S. Pat. No. 4,700,276 (xe2x80x9cFREYMANxe2x80x9d), entitled ULTRA HIGH DENSITY PAD ARRAY CHIP CARRIER. As generally disclosed therein, a ceramic substrate is provided with a plurality of through holes plugged with solder on its bottom surface. These solder plugs (206) are arranged in an array pattern, and form external surface mount interconnection points for the final chip carrier arrangement. The solder plugs are generally hemispherical, and permit the substrate to sit high above the board to which the carrier is mounted. A semiconductor package having an array of solder balls as its interconnection points on an external surface thereof is referred to herein as a Ball Grid Array (BGA) type package.
Generally, BGA solder balls are of two types: (1) eutectic masses that melt upon reflow; and (2) masses such as of 90:10 lead:tin that are not melted, but rather are attached with a eutectic material. The first type of solder ball will collapse slightly (e.g., approximately 6 mils) upon reflow, resulting in some concern over the final planarity of the plurality of connections effected thereby. The second type of solder ball does not collapse, since they are not reflowed. However, since a eutectic material is employed to attach the second type of solder balls, certain substrate materials that cannot withstand the heat associated with eutectic attach processes cannot be employed. This information is provided for general background purposes.
Another type of semiconductor package is the Land Grid Array (LGA), which is provided with a plurality (e.g., an array) of terminals (contact pads (or xe2x80x9clandsxe2x80x9d) on a surface thereof. Generally, resilient interconnection elements are used to make electrical connections to the lands of an LGA. The present invention discloses a xe2x80x9csocketxe2x80x9d having a plurality of resilient interconnection elements for making electrical connections to the terminals of an electronic component such as an LGA-type semiconductor package.
It is generally desired that sockets for LGA and BGA type semiconductor packages be soldered down (e.g., surface-mounted) to a circuit board. Prior art sockets relying on pins require corresponding holes through the circuit board. Using conventional techniques of fabricating holes (e.g., plated through holes) in circuit boards, spacing between adjacent holes (pitch) is typically constrained to no less than 100 mils between adjacent holes. Moreover, plated through holes represent an additional cost in the manufacture of circuit boards. What is needed is a xe2x80x9csolder-downxe2x80x9d or xe2x80x9csurface-mountablexe2x80x9d socket to permit connections to be made at a finer pitch (e.g., 50 mils) and at reduced cost.
Additional references of interest, vis-a-vis BGA and LGA type packages include the following U.S. Pat. Nos. 5,241,133; 5,136,366; 5,077,633; 5,006,673; and 4,700,473.
The aforementioned BGA type package is surface-mounted, by soldering the semiconductor package down onto a PCB. This effects a more-or-less permanent connection of the packaged semiconductor device to the PCB. In order to remove the packaged semiconductor device (such as for replacement or upgrading), it would be necessary to unsolder the entire package from the PCBxe2x80x94a process which can damage either the PCB or the semiconductor device contained within the semiconductor package. Moreover, in order to unsolder a component from a PCB, it is generally necessary to remove the PCB from the system in which it is located.
Techniques for demountably connecting semiconductor packages to PCBs do not suffer from such vagaries. For example, a semiconductor package having pins is readily plugged into a socket which is permanently mounted to a PCB, and is just as readily removed from the socket.
One aspect of the present invention is directed to providing a technique whereby any electronic component such as a BGA or an LGA type semiconductor package can readily be demounted, without unsoldering, from a PCBxe2x80x94in other words, providing xe2x80x9csocketsxe2x80x9d for BGA and LGA type semiconductor packages. This facilitates not only the replacement/upgrading of the packaged semiconductor device, but also provides the opportunity to test the packaged semiconductor device in instances where the PCB is a probe card, or a probe card insert.
As a general proposition, demountable connections require some sort of pressure contact to be made between electronic components. Sockets for receiving pinned semiconductor packages typically have leaf-type spring elements for receiving the package pins.
The following U.S. Pat. Nos. are cited as being of interest: 5,386,344; 5,336,380; 5,317,479; 5,086,337; 5,067,007; 4,989,069; 4,893,172; 4,793,814; 4,777,564; 4,764,848; 4,667,219; 4,642,889; 4,330,165; 4,295,700; 4,067,104; 3,795,037; 3,616,532; and 3,509,270.
Another aspect of the present invention is directed to techniques for forming solder balls and/or raised solder bumps on electronic components, particularly on chip carriers or semiconductor packages. In the main hereinafter, techniques for forming solder xe2x80x9cballsxe2x80x9d are discussed.
Techniques for forming solder balls and/or raised solder bumps on electronic components include, by way of example only:
(1) applying dollops (small quantities) of solder paste to contact pads and reflowing the solder paste;
(2) solder-plugging plated areas (see, e.g., FIG. 2c of FREYMAN);
(3) molding solder ball contacts directly on a substrate (see, e.g., U.S. Pat. No. 5,381,848); and
(4) filling holes in a film carrier with solder, placing the carrier over the substrate, and reflowing the solder to adhere to contact pads on the substrate (see, e.g., U.S. Pat. No. 5,388,327).
Other methods of forming raised solder contacts, of some relevance to the present invention, are the techniques disclosed in the aforementioned commonly-owned, copending U.S. patent applications Ser. Nos. 08/152,812, 08/340,144 and 08/452,255, which generally involve bonding a wire at two (both) ends to a terminal of an electronic component and overcoating the wire with solder. (See, e.g., FIGS. 24A and 24B of Ser. No. 08/452,255; FIG. 16 of Ser. No. 08/340,144; and FIGS. 2-5 of Ser. No. 08/152,812.)
It is a general object of the present invention to provide a technique for fabricating interconnection elements for electronic components.
It is another object of the invention to provide interconnection elements that attach easily to electronic components.
It is another object of the invention to provide interconnection elements that are suitable for making pressure contact to electronic components.
It is another object of the invention to provide a technique for demountably interconnecting (socketing) a BGA-type semiconductor package to an electronic component, such as a PCB.
It is another object of the invention to provide a technique for demountably interconnecting (socketing) an LGA-type semiconductor package to an electronic component, such as a PCB.
It is another object of the invention to provide a technique for forming solder balls and/or raised solder bumps on electronic components, particularly on chip carriers or semiconductor packages.
According to the invention, techniques are disclosed for fabricating interconnection elements, particularly spring elements, and for mounting the interconnection elements to electronic components. The disclosed techniques overcome problems associated with making spring elements of extremely small size, yet capable of exerting contact forces of sufficient magnitude to ensure reliable interconnections. The disclosed techniques also overcome problems associated with mounting springs on electronic components, such as semiconductor devices.
According to the invention, a xe2x80x9ccompositexe2x80x9d (multilayer) interconnection element is fabricated by mounting an elongate element (xe2x80x9ccorexe2x80x9d) to an electronic component, shaping the core to have a spring shape, and overcoating the core to enhance the physical (e.g., spring) characteristics of the resulting composite interconnection element and/or to securely anchor the resulting composite interconnection element to the electronic component.
The use of the term xe2x80x9ccompositexe2x80x9d, throughout the description set forth herein, is consistent with a xe2x80x98genericxe2x80x99 meaning of the term (e.g., formed of two or more elements), and is not to be confused with any usage of the term xe2x80x9ccompositexe2x80x9d in other fields of endeavor, for example, as it may be applied to materials such as glass, carbon or other fibers supported in a matrix of resin or the like.
As used herein, the term xe2x80x9cspring shapexe2x80x9d refers to virtually any shape of an elongate element which will exhibit elastic (restorative) movement of an end (tip) of the elongate element with respect to a force applied to the tip. This includes elongate elements shaped to have one or more bends, as well as substantially straight elongate elements.
As used herein, the terms xe2x80x9ccontact areaxe2x80x9d, xe2x80x9cterminalxe2x80x9d, xe2x80x9cpadxe2x80x9d, and the like refer to any conductive area on any electronic component to which an interconnection element is mounted or makes contact.
As used herein, the term xe2x80x9csolder ballxe2x80x9d refers to any mass of solder, or the like, providing a solderable, raised contact structure on a surface of an electronic component such as a semiconductor package or a support substrate. Such solder balls are employed to make permanent electrical connections between the electronic component to which they are mounted and terminals of another electronic component.
Alternatively, the core is shaped prior to mounting to an electronic component.
Alternatively, the core is mounted to or is a part of a sacrificial substrate which is not an electronic component. The sacrificial substrate is removed after shaping, and either before or after overcoating. According to an aspect of the invention, tips having various topographies can be disposed at the contact ends of the interconnection elements. (See also FIGS. 11A-11F of the aforementioned PARENT CASE.)
In an embodiment of the invention, the core is a xe2x80x9csoftxe2x80x9d material having a relatively low yield strength, and is overcoated with a xe2x80x9chardxe2x80x9d material having a relatively high yield strength. For example, a soft material such as a gold wire is attached (e.g., by wire bonding) to a bond pad of a semiconductor device and is overcoated (e.g., by electrochemical plating) with a hard material such nickel and its alloys.
Vis-a-vis overcoating the core, single and multi-layer overcoatings, xe2x80x9croughxe2x80x9d overcoatings having microprotrusions (see also FIGS. 5C and 5D of the PARENT CASE), and overcoatings extending the entire length of or only a portion of the length of the core, are described. In the latter case, the tip of the core may suitably be exposed for making contact to an electronic component (see also FIG. 5B of the PARENT CASE).
Generally, throughout the description set forth herein, the term xe2x80x9cplatingxe2x80x9d is used as exemplary of a number of techniques for overcoating the core. It is within the scope of this invention that the core can be overcoated by any suitable technique including, but not limited to: various processes involving deposition of materials out of aqueous solutions; electrolytic plating; electroless plating; chemical vapor deposition (CVD); physical vapor deposition (PVD); processes causing the deposition of materials through induced disintegration of liquid or solid precursors; and the like, all of these techniques for depositing materials being generally well known.
Generally, for overcoating the core with a metallic material such as nickel, electrochemical processes are preferred, especially electroless plating.
In another embodiment of the invention, the core is an elongate element of a xe2x80x9chardxe2x80x9d material, inherently suitable to functioning as a spring element, and is mounted at one end to a terminal of an electronic component. The core, and at least an adjacent area of the terminal, is overcoated with a material which will enhance anchoring the core to the terminal. In this manner, it is not necessary that the core be well-mounted to the terminal prior to overcoating, and processes which are less potentially damaging to the electronic component may be employed to xe2x80x9ctackxe2x80x9d the core in place for subsequent overcoating. These xe2x80x9cfriendlyxe2x80x9d processes include soldering, gluing, and piercing an end of the hard core into a soft portion of the terminal.
Embodiments wherein the core is a wire are disclosed. Embodiments wherein the core is a flat tab (conductive metallic ribbon) are also disclosed.
Representative materials, both for the core and for the overcoatings, are disclosed.
In the main hereinafter, techniques involving beginning with a relatively soft (low yield strength) core, which is generally of very small dimension (e.g., 3.0 mil or less) are described. Soft materials, such as gold, which attach easily to semiconductor devices, generally lack sufficient resiliency to function as springs. (Such soft, metallic materials exhibit primarily plastic, rather than elastic deformation.) Other soft materials which may attach easily to semiconductor devices and possess appropriate resiliency are often electrically non-conductive, as in the case of most elastomeric materials. In either case, desired structural and electrical characteristics can be imparted to the resulting composite interconnection element by the overcoating applied over the core. The resulting composite interconnection element can be made very small, yet can exhibit appropriate contact forces. Moreover, a plurality of such composite interconnection elements can be arranged at a fine pitch (e.g., 10 mils), even though they have a length (e.g., 100 mils) which is much greater than the distance to a neighboring composite interconnection element (the distance between neighboring interconnection elements being termed xe2x80x9cpitchxe2x80x9d).
It is within the scope of this invention that composite interconnection elements can be fabricated on a microminiature scale, for example as xe2x80x9cmicrospringsxe2x80x9d for connectors and sockets, having cross-sectional dimensions on the order of twenty-five microns (xcexcm), or less. This ability to manufacture reliable interconnection having dimensions measured in microns, rather than mils, squarely addresses the evolving needs of existing interconnection technology and future area array technology.
The composite interconnection elements of the present invention exhibit superior electrical characteristics, including electrical conductivity, solderability and low contact resistance. In many cases, deflection of the interconnection element in response to applied contact forces results in a xe2x80x9cwipingxe2x80x9d contact, which helps ensure that a reliable contact is made.
An additional advantage of the present invention is that connections made with the interconnection elements of the present invention are readily demountable. Soldering, to effect the interconnection to a terminal of an electronic component is optional, but is generally not preferred at a system level.
According to an aspect of the invention, techniques are described for making interconnection elements having controlled impedance. These techniques generally involve coating (e.g., electrophoretically) a conductive core or an entire composite interconnection element with a dielectric material (insulating layer), and overcoating the dielectric material with an outer layer of a conductive material. By grounding the outer conductive material layer, the resulting interconnection element can effectively be shielded, and its impedance can readily be controlled. (See also FIG. 10K of the PARENT CASE.)
According to an aspect of the invention, interconnection elements can be pre-fabricated as individual units, for later attachment to electronic components. Various techniques for accomplishing this objective are set forth herein. Although not specifically covered in this document, it is deemed to be relatively straightforward to fabricate a machine that will handle the mounting of a plurality of individual interconnection elements to a substrate or, alternatively, suspending a plurality of individual interconnection elements in an elastomer, or on a support substrate.
It should clearly be understood that the composite interconnection element of the present invention differs dramatically from interconnection elements of the prior art which have been coated to enhance their electrical conductivity characteristics or to enhance their resistance to corrosion.
The overcoating of the present invention is specifically intended to substantially enhance anchoring of the interconnection element to a terminal of an electronic component and/or to impart desired resilient characteristics to the resulting composite interconnection element. Stresses (contact forces) are directed to portions of the interconnection elements which are specifically intended to absorb the stresses.
It should also be appreciated that the present invention provides essentially a new technique for making spring structures. Generally, the operative structure of the resulting spring is a product of plating, rather than of bending and shaping. This opens the door to using a wide variety of materials to establish the spring shape, and a variety of xe2x80x9cfriendlyxe2x80x9d processes for attaching the xe2x80x9cfalseworkxe2x80x9d of the core to electronic components. The overcoating functions as a xe2x80x9csuperstructurexe2x80x9d over the xe2x80x9cfalseworkxe2x80x9d of the core, both of which terms have their origins in the field of civil engineering.
According to one aspect of the present invention, xe2x80x9csocketsxe2x80x9d are provided for permitting LGA and BGA type semiconductor packages to be removably connected (socketed) to an electronic component such as a circuit board (e.g., PCB, PWB). Generally, the sockets include a support substrate having a top surface and a bottom surface. Solder balls, or the like, are provided on the bottom surface of the support substrate for soldering the socket to a circuit board, thereby effecting a permanent (albeit demountable) connection between the socket and a circuit board (hence,the term xe2x80x9csolder-downxe2x80x9d, as used herein). A plurality of resilient contact structures are provided on the top surface of the support substrate (or in any suitable manner permitting the resilient contact structures to extend upward from the top surface of the support substrate) for making pressure connections to the external connection points (pads, balls) of an LGA-type or of a BGA-type package, respectively.
Generally, throughout the socket embodiments disclosed herein, any resilient contact structure may be used. The composite interconnection elements of the present invention are simply an example of suitable resilient contact structures for such sockets, and are generally preferred due to their aforementioned relative ease of manufacture with small dimensions.
In an embodiment of the invention serving as a socket for LGA-type packages, the pressure contact is made to tips of the resilient contact structures in a direction which is generally normal to the top surface of the support substrate.
In an embodiment of the invention serving as a socket for BGA-type packages, the pressure contact is made to tips of the resilient contact structures in a direction which is generally parallel to the top surface of the support substrate.
Generally, the embodiments of solder-down sockets described herein provide an effective technique for making pressure connections to terminals of any electronic component, including semiconductor packages and bare unpackaged semiconductor dies. The solder-down socket includes a support substrate having a top surface and a bottom surface, a plurality of resilient contact structures extending from the top surface of the support substrate, each resilient contact structure having a tip at a free end thereof; and means for effecting a pressure connection between the tips of the resilient contact structures and the terminals of the electronic component. Generally, either one or the other of the electronic component or the tips of the resilient contact structures must be moved, relative to the other, to effect such pressure connections. For example, the means for effecting the pressure connection may be a movable sliding element to which the electronic component is mounted, suitable for moving the terminals of the electronic component against the tips of the resilient contact structures. Alternatively, the means for effecting the pressure connection may be a movable sliding element acting upon the resilient contact structures, suitable for moving the tips of the resilient contact structures against the terminals of the electronic component. In either case, it is desirable to effect a wiping movement of the tips of the resilient contact structures against the terminals of the electronic component. Preferably, irrespective of whether it is the tips of the resilient contact structures or the terminals themselves that are moved, a mechanism is provided for limiting how far the tips of the resilient contact structures wide across the terminals of the electronic component, to ensure that they remain in pressure contact with the terminals of the electronic component. As noted, it is preferred that the socket be permanently mounted to a circuit board. To this end, it is preferred that a plurality of solderable raised contact structure are disposed on the bottom surface of the support substrate and connected via the support substrate to the plurality of resilient contact structures.
It should be understood that the LGA-type sockets disclosed herein are suitable for making pressure connections to bare dies having bond pads disposed on a surface thereof, and that the BGA type sockets disclosed herein are suitable for making pressure connections to bare dies having raised contact structures disposed on a surface thereof. An example of raised contact structures on a surface of a semiconductor die are raised solder contacts (bumps) fabricated by IBM""s xe2x80x9cC4xe2x80x9d process. As used herein, a xe2x80x9cbare diexe2x80x9d is a semiconductor chip (device) that has not been packaged, whether the chip is aggregated with other chips on a semiconductor wafer or after individual chips have been singulated from a semiconductor wafer.
Additionally, a novel technique is disclosed for mounting solder balls on pads (contact areas, terminals) of an electronic component. For example, this technique can be employed to mount the aforementioned solder balls on the aforementioned support substrates for LGA and BGA solder-down sockets.
Generally, the solder preform includes a plurality of large solder masses connected to one another by a plurality of smaller solder bridges. The solder preform is disposed against a surface of an electronic component whereupon it is desired to mount solder balls, and the solder preform is heated so as to reflow the solder masses and solder bridges. During reflow, the solder masses become solder balls, and the solder bridges are subsumed into the solder balls. Preferably, soldering flux or solder paste is provided on either the solder preform or on the pads of the electronic component prior to reflow heating.
Other objects, features and advantages of the invention will become apparent in light of the following description thereof.